Several problems are associated with conditioning of air inside cabins of aircrafts currently in operation. For example, inboard pressure in an aircraft flying at a high altitude is made higher than outboard pressure by pressurization, but in consideration for a fuselage strength, the inboard pressure after the pressurization is set lower than a pressure on the earth's surface. More specifically, setting the inboard pressure to a pressure (75.27 kPa) at 2438 m (8000 ft) has been approved for operation. However, in such operation, an oxygen partial pressure of air inside the cabin becomes about ¾ of that on the earth's surface. Furthermore, a humidity of air inside the cabin decreases due to ventilation. Such decrease in oxygen partial pressure and humidity of air inside the cabin decreases the comfort level of passengers.
Accordingly, it has been suggested to separate the air introduced from outside the airplane into oxygen-enriched air and nitrogen-enriched air with a selectively permeable membrane, supply the oxygen-enriched air for air conditioning into a cabin, and increase an oxygen partial pressure of the entire air inside the cabin, or to recover moisture contained in the air flowing out of the cabin and improve humidity of air inside the cabin by reusing this moisture (see Patent Documents 1, 2 and Non-Patent Document 1). It has also been suggested to supply oxygen-enriched air via an oxygen mask to each passenger in the cabin for emergency when the cabin is decompressed or for medical treatment (see Patent Document 3).
Patent Document 1: U.S. Pat. No. 6,655,168.
Patent Document 2: U.S. Pat. No. 6,666,039.
Patent Document 3: U.S. Pat. No. 6,997,970.
Non-Patent Document 1: Saito Hidefumi and three others. “Advanced Air Conditioning System”. Japan Society for Aeronautical and Space Sciences, Japan Aeronautical Engineer's Association. October 2005, Nagoya International Conference Hall, Preprints of 43rd Symposium on Airplane Technology, pages 594-600.